As for construction of a SAW resonator where there is provided a fingers-interleaved electrode (referred to as IDT (Interdigital transducer)) on the surface of a piezoelectric plate such as a quartz piece and a reflector disposed at both ends of this IDT, for example, U.S. Pat. No. 471,616 Official Gazette and U.S. Pat. No. 3,886,504 Official Gazette disclose such construction.
As the piezoelectric plate constituting the SAW resonator, a so-called ST-cut SAW resonator is known which utilizes a crystal ST cut and an X axis thereof serves as the propagating direction of the elastic wave. In this ST cut SAW resonator the zero temperature coefficient that is a primary coefficient .alpha. for the frequency temperature characteristic becomes 0, thus being excellent in frequency stability.
Conventionally, the SAW device has been used as a SAW filter, which is a filter for use with high frequency, so that not so high a Q-value is required. However, in recent years, in a SAW resonator constituting an oscillator for the use, it is important that a further stable resonance frequency be obtained and it is required to provide the resonator with a favorable temperature characteristics and high Q value. Moreover, the primary temperature coefficient .alpha. for the ST cut SAW resonator is almost 0, while the secondary temperature coefficient .beta. is relatively large i.e. -3.4.times.10 .sup.-8 /.degree.C.sup.2. Thus, the frequency change of approximately 110 ppm will be generated in the temperature range of -20.degree. C. through 80.degree. C. which is an operating temperature range of the SAW resonator. Therefore, under usage with the mobile communication, etc., it is necessary to provide a SAW resonator having further stable temperature characteristics in order to generate the high frequency precisely.